In the initial stage of semiconductor industries, bubbling was in wide use where a carrier gas such as oxygen or the like was passed through water in a bubbler. Although this technique was advantageous in that a wide range of a moisture content could be covered, a problem on pollution could not be avoided, and thus, the technique is rarely used at present. Accordingly, an oxygen and hydrogen combustion method, i.e. a pyrogenic system, has been widespread in order to avoid the disadvantage of the bubbler.